The method of the invention is specifically designed for use in what are known as trunked networks, which are typically company networks or government networks in which all channels are shared by several companies or government organizations and in which the subscribers have, in addition to their own subscriber numbers, group numbers indicating the group call group or subscriber group the subscriber belongs to, thus enabling calls intended for members of a predefined group to be switched to subscribers of the group.
A group call is one of the essential functions of a private mobile radio system. A group call is used, for example, in all kinds of operations involving more than one participant, particularly when an entire group must be constantly aware of the progression of events. Typically, subscribers participating in a group call are directed one by one to speak on the same logical radio channel, and in that situation it would be desirable that several subscribers were not switched to speak simultaneously on the same channel.
Traffic in a group call in trunked networks of this kind is nearly always of the semiduplex type, i.e. listening is not possible during transmitting. This affords a significant advantage in that, within the area of one base station, all participants in a call can share the same channel irrespective of the number of participants. Only one subscriber, however, is able to speak at a time. Neither is a subscriber aware upon speaking that another subscriber is speaking at the same time.
In known, particularly analog, trunked radio networks realization of a group call is based on the use of a pressel or PTT (push-to-talk button) as a transmitter switch: by pressing the PTT, the transmitter is switched on, and the subscriber can communicate on the channel. Pressing the PTT also mutes the receiver of the subscriber unit. Each subscriber may attempt to communicate whenever he or she so wishes, and thus it is possible that several subscribers attempt to communicate simultaneously without being aware of one another. If, in a conventional system, more than one participants attempt to speak simultaneously on the same channel, the instantaneous signal strength determines which one will be heard through. If the signals have sufficiently equal power levels, none of them will be distinct enough to be audible. Furthermore, since the receiver of each speaker is muted, the speakers are not aware that the channel they are using is already in use.
The problem becomes more complex if more than one base station is connected to the same group call. In such a case, the normal solution is to add up the speech paths of the base stations in the system, for example by means of a conference connection in the mobile exchange. If two subscribers are speaking simultaneously in the service areas of different base stations, the other subscribers will hear the speech of both parties concurrently. Such a solution is typical of analog networks, but a conference connection can also be employed in digital systems with special arrangements.
The problems presented above can be solved in a digital trunked system, as a solution is known in which concurrent transmission by two subscribers in a group call is prevented, but this occurs only in the area of one base station. Yet the problem exists in practice also between subscribers located within the coverage areas of different base stations, and further between subscribers who are located within the service areas of several exchanges in a trunked network and who belong to the same group call group and participate in the same group call.
The above described prior art solutions are first of all attended by the drawback that concurrent speaking has not been prevented and an individual subscriber cannot be aware that he or she is speaking concurrently with another subscriber until his or her speech item, i.e. speech transmission turn, is over. In the following, a group call executed within the coverage areas of a plurality of base stations is discussed in greater detail. A digital system typically does not employ a conference connection, but the group call is accomplished by switching, similarly as in broadcast, the signal of the currently speaking subscriber, i.e. the speech from the base station involved, to all base stations involved in the call, to be transmitted further to the other group members participating in the group call. The switching is always changed when the speech transmission turn changes, such that the speech or other transmission from the subscriber currently holding the speech transmission turn is sent to all other subscribers participating in the group call.
The solution described above is very functional as long as only one subscriber is speaking at a time. The problem lies in how to proceed if two subscribers are speaking simultaneously within the coverage areas of different base stations. A particular problem is presented by the fact that since only one subscriber at a time is switched through, concurrent speaking is not necessarily detected at all. Neither of the subscribers currently speaking is aware that he or she is speaking concurrently with the other, since listening is not possible during speaking. Nor are the other subscribers aware of the concurrent speaking, since the speech of one subscriber only is switched through. This is a particular problem in government networks and other private mobile radio networks in which vital traffic wherein safety is involved is transmitted, that is, in networks in which it is essential that messages are forwarded to destination swiftly and that the subscriber has certainty that the speech delivered by him or her was received by all members of the group call group.